On Thu, May 14, 2020 at 07:22:50PM -0400, Peter Xu wrote: > On Thu, May 14, 2020 at 03:56:24PM -0700, Sean Christopherson wrote: > > On Thu, May 14, 2020 at 06:05:16PM -0400, Peter Xu wrote: > > > E.g., shm_open() with a handle and fill one 0xff page, then remap it to > > > anywhere needed in QEMU? > > > > Mapping that 4k page over and over is going to get expensive, e.g. each > > duplicate will need a VMA and a memslot, plus any PTE overhead. If the > > total sum of the holes is >2mb it'll even overflow the mumber of allowed > > memslots. > > What's the PTE overhead you mentioned? We need to fill PTEs one by one on > fault even if the page is allocated in the kernel, am I right? It won't require host PTEs for every page if it's a kernel page. I doubt PTEs are a significant overhead, especially compared to memslots, but it's still worth considering. My thought was to skimp on both host PTEs _and_ KVM SPTEs by always sending the PCI hole accesses down the slow MMIO path[*]. [*] https://lkml.kernel.org/r/20200514194624.GB15847@xxxxxxxxxxxxxxx > 4K is only an example - we can also use more pages as the template. However I > guess the kvm memslot count could be a limit.. Could I ask what's the normal > size of this 0xff region, and its distribution? > > Thanks, > > -- > Peter Xu >
